Zinc Oxide varistors are finding increasing usage as replacements for silicon carbide voltage surge protection devices. Silicon carbide devices however, require series voltage gaps to prevent the silicon carbide material from being damaged during a voltage overload condition. With zinc oxide devices it is theoretically possible to substitute the zinc oxide resistance elements for the silicon carbide element without the series voltage gap. To date the use of zinc oxide varistors without sparkgaps has not proven feasible because of the change in the electrical characteristics of the zinc oxide varistor under subjection to a continuous source of A.C. potential. The continuous application of A.C. voltage to the zinc oxide varistor causes the leakage current through the zinc oxide to increase over a period of time. The increase in the zinc oxide varistor leakage current beyond a relatively low value may cause disc failure by the mechanism of thermal runaway at normal operating voltages.
For the purpose of this disclosure the increase in the varistor leakage current under the influence of A.C. voltage is defined as "A.C. drift".
U.S. Pat. No. 3,928,245 issued Dec. 23, 1975, describes one method for manufacturing an improved zinc oxide varistor having decreased A.C. drift. The improvement is believed to be due to the addition of the oxides of barium and boron to the basic zinc oxide and bismuth oxide compositions also containing silicon oxide. Varistors manufactured using the described oxide additives showed fairly stable leakage current values out to 200 hours of operation before the leakage currents began to increase under the influence of the continuously applied A.C. voltage.
U.S. Pat. No. 4,046,847 filed Dec. 2, 1975 and entitled "Process for Improving the Stability of Sintered Zinc Oxide Varistors" discloses a method for further improving zinc oxide varistor leakage current stability out to approximately 900 hours before the varistor leakage current begins to increase. The aforementioned method treats the sintered zinc oxide varistor by cooling the varistor immediately after sintering to a temperature below 400.degree. C. and reheating the varistor up to a maximum of 700.degree. C., recooling the varistor down to 400.degree. C. and recycling the reheating and recooling process.
It has been discovered that zinc oxide varistors can be rendered stable over extended periods of time without the need for recycling when the time and temperature parameters are carefully chosen for a particular resistor composition and configuration. It has also been discovered that the maximum effective temperature is in the order of 800.degree. C. rather than 700.degree. C.